Within the last two decades there has been a substantial shift in demand away from traditional explosive compositions such as TNT, dynamite, and nitroglycerin for hard rock mining, excavation, and similar commercial blasting, in favor of cheaper, readily available inorganic oxygen-supplying salts such as prilled ammonium nitrate (AN) or mixtures of such salt(s) with an organic fuel or oil (ANFO).
Such formulations are relatively inexpensive, can be manufactured "in situ" with greater safety than most traditional explosives, and also avoid a number of problems generally associated with explosive storage and transportation in the field.
If a bore hole is to be very deep and/or wet, however, AN and ANFO formulations are less attractive, since they are easily desensitized and require protection (a) by special packaging with attendant cost and oxygen-balance problems, or (b) by the addition of expensive coatings and the like. Moreover, ANFO has a limited volume energy due to its relatively low bulk density.
The burden of some of the above-listed problems is softened by using explosive slurries having high bulk densities, but such formulations are still not water proof, and must rely heavily upon gassification or other known density control means to retain even a minimally acceptable level of sensitivity. Moreover, pressure conditions within deep bore holes can cause compression of gas bubbles, resulting in retention of unexploded charges in difficult-to-reach areas where further drilling, mining or excavation may be necessary.
One substantial breakthrough with respect to the problem of moisture resistance is described, for instance, in U.S. Pat. No. 3,161,551 of Egly. Here a water-resistant blasting agent is obtained by combining at least one solid prilled inorganic oxygen supplying salt such as ammonium nitrate (AN) with an invert emulsion containing 50-70% aqueous ammonium nitrate (AN) solution as the discontinuous phase and a carbonaceous fuel oil plus organic emulsifier such as a long chain fatty acid or ester derivatives thereof, as the continuous external hydrophobic phase.
Egly's class of compositions exhibit improved resistance to water because the water-in-oil emulsion has the capacity to fill natural voids in the solid prilled inorganic salt component and water cannot easily force its way through the continuous external hydrophobic emulsion phase. Balanced against such advantage, however, are serious sensitivity and storage problems which appear to be due, in part, to a tendency of the solid oxidizer salt component to promote or encourage desensitizing crystallization within the discontinuous aqueous phase during storage.
Additional variations in the use of oxidizer salts in invert emulsion explosives are demonstrated, for instance, in Wade (U.S. Pat. Nos. 4,149,916; 4,110,134) Sudweeks (U.S. Pat. No. 4,322,258) and Jessop (U.S. Pat. No. 4,356,044). Generally speaking, such compositions also tend to be deficient in sensitivity characteristics.
Bluhm (U.S. Pat. No. 3,447,978) represents an attempt to avoid such lack of sensitivity by the use of at least 4% by volume of occluded or entrained gas within an invert emulsion system. Bluhm's compositions consist essentially of
(1) a known water-in-oil emulsifier;
(2) a discontinuous ammonium nitrate aqueous phase (optionally supplemented by other water soluble oxidizer salts) within a continuous organic phase consisting of a carbonaceous fuel having the required predetermined gas-retaining consistency at 70.degree. F.; and
(3) a functionally important amount of occluded gas such as air in the form of bubbles or hollow glass globules and the like, as density control agents.
Such explosive compositions, while capable of avoiding a number of deep wet-bore hole problems, suffer from deficiencies in stability and sensitivity and face potential pollution problems unless the proportion by weight of discontinuous aqueous phase-to-continuous organic phase is kept high enough to approximate an Oxygen Balance. Moreover, the low density of such compositions results in less than optimal volume energy value ranges.
In the Tomic Patent (U.S. Pat. No. 3,770,522), gas bubbles and other density control agents such as microballoons are proposed along with use of a stearate salt as emulsifier, plus aluminum, magnesium, and smokeless powder as supplementary fuels. Such use of supplementary fuels, however, does not lead to a high volume-energy range as desired.
Efforts to lessen or eliminate density control agents to increase volume energy and reduce cost of production have not been satisfactory because of continued substantial difficulty in controlled firing of such compositions. Moreover, the use of known temperature-sensitive explosive sensitizers such as nitroglycerin tend to improve sensitivity at the expense of safe handling characteristics normally expected of emulsion blasting agents. Sensitizer compositions of later type are described, for instance, in Berthmann et al (U.S. Pat. No. 3,356,547).
It is an object of the present invention to optimize volume energy and sensitivity of high density invert emulsion blasting compositions while retaining the excellent formulation, handling, and safety characteristics generally attributable to water-in-oil emulsion blasting compositions by adding an effective amount of a sensitizing formulation.
It is a further object to optimize volume-energy release characteristics and resistance to deep hole pressure effects of high density invert emulsion containing blasting compositions while retaining sensitivity.